1. Field of the Invention
The apparatus of the present invention relates to electrical power distribution systems, and more specifically to a detection circuit for a ground fault circuit interrupter, having a solenoid with a moveable core.
2. Description of the Prior Art
Ground fault circuit interrupters (GFCI) are devices which can be mounted in standard electrical receptacle boxes, which can be incorporated into circuit breakers, or which can be portable stand alone units. GFCI's are useful for quickly interrupting the flow of current when a fault occurs. The ground fault circuit interrupter is typically comprised of an electronic circuit for detecting the electrical fault and an electromechanical current interrupter. When the cause of the fault has been corrected, the ground fault circuit interrupter can be reset by depressing a reset button disposed on the face of the ground fault interrupter. A representative example of such a device is described in U.S. Pat. No. 3,813,579 by Doyle et al., issued on May 28, 1974.
There are several patents which disclose ground fault circuit interrupters, The electromechanical current interrupter of these devices, however, can be characterized as an electro-mechanical device utilizing either a moving core and helical coil as the power element or as an electromechanical device utilizing a fixed core, helical coil and an armature as the power element. A typical example of a ground fault circuit interrupter device having a moving core which opens spring actuated contacts is disclosed in U.S. Pat. No. 4,247,840 Cooper et al., issued Jan. 21, 1981 and assigned to GTE. A typical example of a ground fault interrupter device having a fixed core which opens spring actuated contacts is disclosed in U.S. Pat. No. 4,086,549, issued Apr. 25, 1978, and assigned to assignee of the present invention.
Copending U.S. patent application Ser. No. 690,160, filed Jan. 10, 1985, and also assigned to the assignee of the present invention discloses a ground fault circuit interrupter of the type having a solenoid with a moving core that does not require the assistance of springs to separate a pair of contacts. The ground fault circuit interrupter includes two pivot arms or cam actuators which are coupled together by a catcher and which rotate when the moveable core displaces the catcher. As the cam actuators rotate, they cause a pair of flexure arms having moveable contacts disposed thereon to deflect. The deflection of the flexure arms causes an opening between stationary contacts and the moveable contacts on each of the flexure arms. The solenoid is energized and the contacts are opened when an electronic circuit detects a fault in the conducting wires connected to the ground fault circuit interrupter. In a conventional ground fault circuit interrupter detection circuit, a circuit integrator is used to activate the moving core of the solenoid. In some instances, such as in the case of the above described copending patent application Ser. No. 690,160, the charging of the integrator to the threshold causes a thyristor to fire to energize the solenoid core. For a low level trip fault current, the charging may occur towards the end of the power line cycle during a low energy interval. Therefore, the current interrupter may not latch open in this interval, thereby allowing the unlatched contacts to close again. Under certain circumstances, this condition can cause the solenoid to fire multiple times before the contacts finally latch open.
Accordingly, there is a need for an improved electronic detection circuit for a ground fault interrupter having a solenoid with a moveable core.